Had to go with NWN on this, but not because the boxed campaigns were good. For me, NWN gets the vote because of the sheer number of hours I enjoyed with that game playing in RP persistant worlds. That game provided the PnP campaigning that I had so missed over the years. And not only that, but playing NWN in that way was the very first time a computer RPG allowed me to create and develop my characters as I wanted to - I could give them full personalities, goals, motives, quirks and so on, and really act on them.
The boxed campaigns I never finished, and I haven't really spent much time with NWN2 yet so I'm only counting NWN, SoU and HoTU here along with the tons of user mods, haks, scripts etc. But for those alone it's worth a vote.

I have fond memories of the Gold Box games, as that was the first time I could play D&D on the computer in full, orchestrated campaigns, which was a far cry from the ramshackle ad-hoc super-casual tabletop games I always had. Those games gave the full-on D&D experience at the time. But the greater flexibility that NWN provided is why I give that the nod over the Gold Box series.

— You know the very powerful and the very stupid have one thing in common. They don't alter their views to fit the facts, they alter the facts to fit their views, which can be uncomfortable if you happen to be one of the facts that needs altering.